White background with filmic blender
Filmic blender uses  a different scale than sRGB, it is designed for a much wider dynamic range, so to get white it is necessary to increase the intensity of the light for that background.
The standard sRGB transform will display &quot;white&quot; when the scene values reach a value of 1

Using the filmic blender transforms, you need a much higher value. &quot;White&quot; is at a value of 16.19.

To visualize how the scene referred values are being mapped using the filmic blender you can use the false colour view transform.
To understand what filmic blender does and how it maps the scene's values to the displayed image please read the following link: Render with a wider dynamic range in cycles to produce photorealistic looking images
and https://github.com/sobotka/filmic-blender